1. Field of the Invention
The present invention relates to a hydraulic anti-skid apparatus for installation in a vehicle braking system between a master cylinder and wheel brake cylinders to prevent the road wheels of the vehicle from locking in braking operation.
2. Discussion of the Background
As one of such anti-skid apparatus as described above, there has been proposed an anti-skid apparatus which comprises a cut-off valve disposed within a braking circuit connecting a master cylinder to a wheel brake cylinder, a bypass valve disposed within a bypass passage of the braking circuit, first and second pistons respectively arranged to control opening and closing operations of the cut-off valve and the bypass valve, the first and second piston each being arranged to be applied at one end thereof with a hydraulic braking pressure from the master cylinder and at the other end thereof with a hydraulic power pressure from a fluid pump, a solenoid valve arranged to be activated when a road wheel of the vehicle tends to be locked in braking operation, the solenoid valve being adapted to apply the hydraulic power pressure to the first piston in its deenergized condition and to connect the first piston to a fluid reservoir in its energized condition, and a regulator valve arranged to control the hydraulic power pressure applied to the first and second pistons in accordance with the hydraulic braking pressure applied thereto from the master cylinder.
In the above-described conventional anti-skid apparatus, the fluid pump is in the form of a fluid pump of large capacity which is driven by a prime mover of the vehicle to apply a hydraulic power pressure to a power-assisted steering apparatus. For this reason, in the case where the anti-skid apparatus is adapted to an automotive vehicle without provision of such a power-assisted steering apparatus, the power of the prime mover is always consumed for operation of the fluid pump during travel of the vehicle. In such a case, it is desirable that the fluid pump is replaced with a fluid pump of small capacity arranged to be driven by an electric motor only when the vehicle is braked. However, if the master cylinder is actuated prior to activation of the fluid pump in the conventional anti-skid apparatus, the first and second pistons are displaced from their initial positions by the hydraulic braking pressure to cause abnormal operations of the cut-off valve and the bypass valve. Furthermore, in the conventional anti-skid apparatus, the first piston is moved backward from the initial position by the hydraulic braking pressure when connected to the fluid reservoir in response to energization of the solenoid valve, and subsequently the first piston is rapidly supplied with pressurized fluid from the regulator valve in response to deenergization of the solenoid valve. During such movement of the first piston, the hydraulic power pressure in the regulator valve is instantaneously decreased, and in turn, the regulator valve is supplied with pressurized fluid from the master cylinder. This causes pressure pulsation in the regulator valve and the master cylinder, resulting in unpleasant shocks on the brake pedal. If in such a situation, the second piston is displaced by the hydraulic braking pressure due to decrease of the hydraulic power pressure, there will occur leakage of the braking fluid across the bypass valve. As a result, the first piston may not be moved to the initial position due to the braking pressure applied thereto across the bypass valve.